Unauthorized reading of card data, such as data encoded on a magnetic stripe card, while the card is being used (hereafter “card skimming”), is a known type of fraud. Card skimming is typically perpetrated by adding a magnetic read head (hereafter “alien reader”) to a fascia of an automated teller machine (ATM) to read a magnetic stripe on a customer's card as the customer inserts or (more commonly) retrieves the card from an ATM. The customer's personal identification number (PIN) is also ascertained when the customer uses the ATM. Examples of how this is achieved include: a video camera that captures images of the PINpad on the ATM, a false PINpad overlay that captures the customer's PIN, or a third party watching the customer (“shoulder surfing”) as he/she enters his/her PIN. The third party can then create a card using the card data read by the alien reader, and can withdraw funds from the customer's account using the created card and the customer's PIN (ascertained by one of the ways described above).
Various methods have been proposed to defeat this type of fraud. One method involves transmitting an electromagnetic signal (hereafter a “jamming signal”) when the card is being transported so that the alien reader cannot detect the magnetically encoded data because of the presence of the jamming signal. Although this technique can be effective, it is possible to use signal processing to cancel out a jamming signal by using another alien reader that receives only the jamming signal and uses this as a reference signal. The reference signal is used to cancel out the jamming signal by subtracting the reference signal from the composite signal (comprising the reference signal and the magnetic signal representing account data from the data card) to reveal the account data signal.
It would be advantageous to be able to prevent or mitigate filtering out of the jamming signal.